Generally, a pressure ring and an oil ring are attached to a piston for an internal combustion engine as piston rings. The pressure ring is provided with a function of preventing a phenomenon (blow-by) in which high pressure combustion gas flows from a combustion chamber toward a crankcase. On the other hand, the oil ring is mainly provided with a function of suppressing a phenomenon (oil loss via the piston ring) in which redundant lubricant on a cylinder inner wall enters a combustion chamber from a crankcase and is consumed. As a combination of conventionally standard piston rings, there is known a combination of three piston rings, i.e., one oil ring and two compression rings including a top ring and a second ring.
In recent years, with reduction tendency in weight and increase tendency in output of the internal combustion engine, quality required for the piston ring is increasing.
One performance required for the piston ring for the internal combustion engine is wear resistance. As a means that enhances the wear resistance, at present, a nitride layer is formed by a nitriding treatment or a hard film is formed by a PVD method on a sliding surface of an outer periphery of the piston ring.
The piston ring for the internal combustion engine is also required to have oil scraping performance for suppressing the oil loss via the piston ring. As a means that enhances the oil scraping performance, it is necessary to form an edge portion mainly on the side of a lower surface of the piston ring into a sharp shape.
When the two requirements are to be satisfied at the same time, however, the hard film is formed on the sliding surface of the outer periphery of the piston ring and the edge portion of the hard film is formed into the sharp shape. According to this, since the hard film formed on the sliding surface is “hard”, the film easily becomes chipped and therefore, especially the edge portion easily becomes chipped.
To solve such a problem, patent document 1 discloses a technique in which the hard film is partially formed instead of covering the entire sliding surface of the piston ring with the hard film, a sharply shaped piston ring base material is exposed in the edge portion which satisfies the oil scraping performance.